The fluidized catalytic cracking of hydrocarbons is the mainstay process for the production of gasoline and light hydrocarbon products from heavy hydrocarbon charge stocks such as vacuum gas oils or residual feeds. Large hydrocarbon molecules associated with the heavy hydrocarbon feed are cracked to break the large hydrocarbon chains thereby producing lighter hydrocarbons. These lighter hydrocarbons are recovered as product and can be used directly or further processed to raise the octane barrel yield relative to the heavy hydrocarbon feed.
The basic equipment or apparatus for the fluidized catalytic cracking of hydrocarbons include a reactor, a regenerator, and a catalyst stripper. The reactor includes a contact zone where the hydrocarbon feed is contacted with a particulate catalyst and a separation zone where product vapors from the cracking reaction are separated from the catalyst. Further product separation takes place in a catalyst stripper that receives catalyst from the separation zone and removes entrained hydrocarbons from the catalyst by counter-current contact with steam or another stripping medium.
The FCC process is carried out by contacting the starting material—generally vacuum gas oil, reduced crude, or another source of relatively high boiling hydrocarbons—with a catalyst made up of a finely divided or particulate solid material. The catalyst is transported like a fluid by passing gas or vapor through it at sufficient velocity to produce a desired regime of fluid transport. Contact of the oil with the fluidized material catalyzes the cracking reaction. The cracking reaction deposits coke on the catalyst. Coke is comprised of hydrogen and carbon and can include other materials in trace quantities such as sulfur and metals that enter the process with the starting material. Coke interferes with the catalytic activity of the catalyst by blocking active sites on the catalyst surface where the cracking reactions take place. Catalyst is traditionally transferred from the stripper to a regenerator for purposes of removing the coke by oxidation with an oxygen-containing gas. An inventory of catalyst having a reduced coke content relative to the catalyst in the stripper, hereinafter referred to as regenerated catalyst, is collected for return to the reaction zone. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas. The balance of the heat leaves the regenerator with the regenerated catalyst. The fluidized catalyst is continuously circulated from the reaction zone to the regeneration zone and then again to the reaction zone. The fluidized catalyst, as well as providing a catalytic function, acts as a vehicle for the transfer of heat from zone to zone. Catalyst exiting the reaction zone is spoken of as being spent, i.e., partially deactivated by the deposition of coke upon the catalyst. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
Refining companies are under increased pressure to reduce CO2 emissions as a result of carbon tax legislation and other drivers such as a desire to demonstrate long-term sustainability. One way of reducing overall CO2 emissions is by improving the energy efficiency of the process. Thus, there is a need to provide a way to improve the overall energy efficiency of a fluid catalytic cracking unit. In order to achieve this end, some prior art systems have proposed recovering power from the hot flue gas. With traditional FCC power recovery technology, however, there are current limitations to increase the FCC flue gas temperature and flowrate using the flue gas power recovery arrangement in which a third stage separator is followed by a power recovery expander. The temperature at the inlet to the expander is restricted by the temperature limit of the FCC catalyst and the regenerator, which determines the expander conditions. Thus current technology is limited on the amount of power that can be produced in the FCC power recovery expander.
In commonly-assigned U.S. Pat. No. 7,802,435, a new flow scheme was proposed by James F. McGehee, which is to install a combustor in the front of turbine expander. The combustor and the expander are separate equipment. By combusting the FCC regenerator flue gas in the combustor, the flue gas temperature can be increased at least 900° C. Heated flue gas enters the expander first and then goes to the steam generator downstream. At the same time, flue gas flow rate increases significantly due to air and fuel added in the combustor. The direct consequence is increased power generation from the turbine expander. Although McGehee's patent aims to increase power recovery, steam generation can be increased as well due to much increased heat content of the flue gas via increased flue temperature and flowrate.
The combustor used in such power recovery applications should be innovated for its placement in front of the expander. To date, the prior art is devoid of any combustor designs that are specially adapted for use in FCC power recovery applications. In other words, a special combustor design is desirably obtained in order for practical applications of the concept of installing a combustor before the expander. The described embodiments of present disclosure aim to meet at least this need. Furthermore, other desirable features and characteristics of the described embodiments will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.